1) Is it possible to make crafted Tantrum bow with 2 "on striking" modifications like: arrow+knives, or knives+time strike?
If you craft one with any on-striking proc besides Arrow, you can use Warbringer UMO to add 2% Chance to cast level 5 Arrow on striking (per orb). So, yes you can have 2 procs (in all fairness I haven't tested this, so no idea how well it'll work)